A conventional valve timing adjusting apparatus, which adjusts a valve timing of intake valves of an internal combustion engine, is disclosed in patent document, JP 2009-62837 A (US 2009-0058344 A1). This valve timing adjusting apparatus includes an electric motor and plural switching elements. The motor is formed of a rotor part, to which a motor shaft is fixed, and a stator coil provided around the rotor part. The switching elements are connected to the stator coil. The valve timing adjusting apparatus further includes a power supply driving part and a phase adjusting mechanism. The power supply driving part switches over switching elements, which are selected to be tuned on at every angular rotation interval of the motor shaft. The phase adjusting mechanism adjusts a relative phase between a crankshaft and a camshaft of the internal combustion engine in accordance with operation states of the internal combustion engine and a rotation of the motor shaft.
When the motor shaft rotates under a state that a current flows in the stator coil and the stator coil generates a magnetic field, an induced voltage is generated in the stator coil. In a case that a target rotation direction of the motor shaft is the same as an actual rotation direction of the motor shaft, the induced voltage is generated in a direction opposite to a voltage supplied to the stator coil by the selected switching elements, which are turned on. As a result, a current flows in the selected switching elements, which are in the on-states, in correspondence to a difference between the supplied voltage and the induced voltage. However, in a case that the target rotation direction of the motor shaft is different from, that is, opposite to the actual rotation direction of the motor shaft, the induced voltage is generated in the same direction as the voltage supplied to the stator coil by the switching elements, which are turned on. As a result, large current flows in the selected switching elements in correspondence to a sum of the supplied voltage and the induced voltage. This large current is likely to generate heat excessively.
According to the valve timing adjusting apparatus disclosed in the patent document, in a case that the target rotation direction of the motor shaft is the same as the actual rotation direction of the motor shaft, the power supply driving part sets an entire range of rotation angle of the motor shaft, which corresponds to 30° in mechanical angle (12° in electrical angle), as an on-state range, in which the selected elements are turned on continuously. In a case that the target rotation direction and the actual rotation direction are different, the power supply driving apart sets the rotation angle range by dividing it into the on-state range and an off-state range, in which the selected elements are turned off continuously. Thus, the voltage is supplied to the stator coil in the on-state range but not supplied in the off-state range. As a result, a period of time, in which the current corresponding to the sum of the supplied voltage and the induced voltage, is shortened thereby to suppress excessive heat generation of the selected elements.
In one exemplary valve timing adjusting apparatus according to the patent document, the motor is mechanically coupled to the crankshaft through the phase adjusting mechanism. When the crankshaft is in rotation as a result of combustion in the internal combustion engine, the motor shaft of the motor also rotates irrespective of a control torque (rotation torque) generated by the power supply to the stator coil. Even when the crankshaft continues to rotate by inertia after stopping of the combustion, the motor shaft continues to rotate in the same direction as the crankshaft rotates while decreasing its rotation speed. The rotor part fixed to the motor shaft has permanent magnets. Since metallic parts such as the stator coil are present around the permanent magnets, magnetic force is generated between the metallic parts and the permanent magnets. This magnetic force exerts on the motor shaft as a braking torque, which impedes rotation of the rotor part.
When the motor shaft rotates with the crankshaft by inertia and its rotation force decreases closely to a stop, the braking torque tends to reverse the rotation direction of the motor shaft momentarily before stopping. When the rotation direction is reversed before stopping as described above, the power supply driving part erroneously determines that the actual rotation direction is different from the target rotation direction and stops its operation. The power supply driving part thus sets the rotation angle range by dividing it into the on-state range and the off-state range, when the target rotation direction and the actual rotation direction are different. When the rotation angle of the motor shaft at the time of motor stopping is in the on-state range, the power supply driving part can drive the motor to generate the rotation torque of the motor shaft at the time of restarting the internal combustion engine. By thus rotating the motor shaft, the phase of the camshaft relative to the crankshaft can be adjusted to a phase, at which a quantity of air compressed in a combustion chamber formed by a cylinder and a piston becomes suitable for restarting the internal combustion engine. When the rotation angle of the motor shaft at the time of motor stopping is in the off-state range, the power supply driving part cannot drive the motor to generate the rotation torque of the motor shaft at the time of restarting of the internal combustion engine. Since the motor shaft cannot be rotated, the phase of the camshaft relative to the crankshaft cannot be adjusted to a phase, which is suitable for restarting the internal combustion engine.
In the other exemplary valve timing adjusting apparatus of different configuration according to the patent document, the power supply driving part sets the entire range of variation, in which the rotation angle range is varied in an advanced direction or retarded direction by a mechanical angle of 15° (electrical angle of 60°), as the on-state range, when the target rotation direction and the actual rotation direction are different. By this setting, the induced voltage generated by the stator coil is decreased and the current corresponding to the sum of the supplied voltage and the induced voltage is decreased. Thus, the excessive heat generation of the selected element is suppressed. In the other exemplary motor control apparatus, the current flows in the stator coil continuously irrespective of the rotation angle range even when the target rotation direction and the actual rotation direction are different. As a result, the motor can be restarted.
In the other exemplary motor control apparatus, differently from the exemplary motor control apparatus, the current corresponding to the sum of the supplied voltage and the induced voltage flows continuously although decreased, when the target rotation direction and the actual rotation direction are different. As a result, suppression of the heat generation of the selected element is limited.